1. Field of the Invention
This invention relates to gaseous discharge lamps which ignite at voltages that are much higher than their operating voltages and, in particular, to the igniting of such lamps.
2. Description of Related Art
Common characteristics of a gaseous discharge lamp are its negative resistance and high igniting voltage. A circuit arrangement for powering such a lamp typically includes a current limiting means, such as a ballast, to compensate for the negative resistance, and often includes circuitry for generating high-voltage pulses to ignite the lamps. Such pulse-generator circuitry typically includes a voltage-sensitive switch (e.g. a sidac) for effecting the continual production of the high-voltage pulses until the lamp ignites. Upon ignition, the voltage across the lamp decreases from a higher open-circuit voltage (OCV) to a lower voltage, which causes the switch to change to a non-conducting state and to effect termination of pulse production. Such a circuit arrangement may also include timer circuitry for limiting the time period during which the high-voltage ignition pulses are applied to the lamp. Such timer circuitry typically includes another switch (e.g. a triac) for controlling the production of the high-voltage pulses independently of the pulse generator circuitry.
FIG. 1 illustrates a generalized example of known circuit arrangements of this type. Such circuit arrangements typically include a ballast B, an ignitor 12 and a gaseous discharge lamp L. The ballast includes input terminals T1 and T2 for connection to a power source (e.g. to a 120 VAC line). It further includes output terminals T3 and T4, for supplying power to the lamp L, and a terminal T5. The ignitor 12 includes a pulse generator 120 and a timer 124. The pulse generator is electrically connected to a conductor C, which carries current to the lamp, for applying high-voltage pulses to the lamp to effect ignition. An input of the timer 124 is electrically connected to the terminal T5 for detecting application of power to the lamp L. An output of the timer is electrically connected to the pulse generator 120 for controlling its activation.
Note that FIG. 1 is a functional block diagram. That is, each block represents a function, but does not necessarily indicate where the elements used to perform that function are located. They may be separately grouped in accordance with function to facilitate the use of plug-in modules. Alternatively, the circuit elements may be distributed to achieve certain other advantages, such as space conservation or temperature distribution. For example, the pulse generator 120 may include a low-impedance pulse-producing winding that is electrically connected in series with the conductor C. This winding may be a separate device or may physically form part of a transformer which is included in the ballast B.
Note further that a circuit arrangement of the type shown in FIG. 1 also includes or utilizes a power supply (not shown), such as a full-bridge rectifier, for converting AC voltage from the power source to DC voltage for powering the circuitry in the ignitor 12. For a specific example of a circuit arrangement of the above-described type, see U.S. Pat. No. 5,424,617.
In operation, the pulse generator 120 applies high-voltage igniting pulses to the lamp L for a predetermined period of time after power is applied via the ballast B. This time period is measured by the timer 124 and is generally equal to the maximum expected time needed to ignite the type of lamp with which the ignitor 12 is to be used. At the end of the predetermined time period, the timer disables the pulse generator. Such disablement is intended to prevent continual production of high-voltage ignition pulses when a lamp is non-functional or when no lamp is present in the circuit.
While such timer circuit arrangements perform the important function of protecting against excessive high-voltage pulse generation, they typically have one or more of the following shortcomings:
Such circuit arrangements continually reignite (or attempt to reignite) lamps which are near their end of life. This undesirable trait, commonly called xe2x80x9ccyclingxe2x80x9d, both stresses the circuitry and lessens the likelihood of timely detecting and replacing end-of-life lamps. This is a common problem with certain types of gaseous-discharge lamps, such as high-pressure sodium (HPS) lamps, which have operating voltages that increase substantially with age.
The circuit arrangement may inactivate the pulse generator before the lamp has warmed up adequately to remain ignited.
If power to an operating lamp is momentarily interrupted, the interruption may be long enough to extinguish the lamp but too short to enable reset of the timer. In this situation, the timer will not run at all or will provide less than the predetermined time needed to re-ignite the lamp.
It is an object of the invention to provide a method and an apparatus for igniting a gaseous discharge lamp in a manner which obviates the above-described shortcomings.
In accordance with the invention, the application of igniting pulses to the lamp is controlled in response to both time and upper and lower threshold voltages. The application of igniting pulses is enabled if:
igniting pulses have not been continuously applied to the lamp for an elapsed time exceeding a predetermined time period; and
the lamp voltage is either above the upper threshold voltage or below the lower threshold voltage.
Lamp voltages above the upper threshold voltage indicate that the lamp has not ignited. Lamp voltages below the lower threshold voltage are too low to ensure that the lamp has become fully ignited. Lamp voltages below the lower threshold typically occur when a starting lamp has not fully warmed up.
Igniting of a gaseous discharge lamp in accordance with the invention provides a means for preventing the continued application of igniting power to non-functional or missing lamps and also for inhibiting xe2x80x9ccyclingxe2x80x9d of end-of-life lamps. In either case, the detected lamp voltage will remain above the upper threshold voltage while igniting pulses are applied for longer than the predetermined time period. This is achieved by adjusting the upper threshold voltage and the predetermined time period to values that correspond to an age which is deemed to be a lamp""s useful end-of-life. Further, by enabling the application of igniting pulses even while the lamp voltage is below the lower threshold voltage, the continued generation of such pulses will be permitted if a lamp that has not warmed up falls out of ignition.
In accordance with another feature of the invention, a timer for measuring the elapsed time is reset whenever the lamp voltage decreases from a voltage above the upper threshold voltage to a voltage below the upper threshold voltage. This ensures that the timer will allow the full predetermined time period to elapse if power is subsequently interrupted, regardless of the brevity of the interruption.